First Evidence for cos 2β > 0 and Resolution of the Cabibbo-Kobayashi-Maskawa Quark-Mixing Unitarity Triangle Ambiguity

Abstract

We present first evidence that the cosine of the CP-violating weak phase 2β is positive, and hence exclude trigonometric multifold solutions of the Cabibbo-Kobayashi-Maskawa (CKM) Unitarity Triangle using a time-dependent Dalitz plot analysis of B^0 → D(*)h^0 with D → K^0_Sπ+π− decays, where h^0 ∈ {π^0,η,ω} denotes a light unflavored and neutral hadron. The measurement is performed combining the final data sets of the BABAR and Belle experiments collected at the Υ(4S) resonance at the asymmetric-energy B factories PEP-II at SLAC and KEKB at KEK, respectively. The data samples contain (471±3) × 10^6BB pairs recorded by the BABAR detector and (772±11) × 10^6BB pairs recorded by the Belle detector. The results of the measurement are sin2β = 0.80±0.14(stat)±0.06(syst)±0.03(model) and cos2β = 0.91±0.22(stat)±0.09(syst)±0.07(model). The result for the direct measurement of the angle β of the CKM Unitarity Triangle is β = [22.5±4.4(stat)±1.2(syst)±0.6(model)]°. The measurement assumes no direct CP violation in B^0 → D(*)h^0 decays. The quoted model uncertainties are due to the composition of the D^0 → K^0Sπ+π− decay amplitude model, which is newly established by performing a Dalitz plot amplitude analysis using a high-statistics e+e− → cc data sample. CP violation is observed in B^0 → D(*)h^0 decays at the level of 5.1 standard deviations. The significance for cos2β > 0 is 3.7 standard deviations. The trigonometric multifold solution π/2 – β = (68.1±0.7)° is excluded at the level of 7.3 standard deviations. The measurement resolves an ambiguity in the determination of the apex of the CKM Unitarity Triangle.

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